Current sensitive circuit protection system

ABSTRACT

An electrical control system which is utilized for opening a circuit breaker which is connected to a load to be protected. The control system comprises a unitary current sensor module which senses current flowing in the load to be protected. The sensor is adapted to have connected at the output thereof a load resistor which is capable of cooperating with the sensed current to provide a range of voltage over which a set of plug-in modules are operable to either provide an indication of characteristics associated with the current or the load or to provide an output function which is adapted to open the circuit breaker. The load resistor is replaceable with other load resistors so that the voltage range may be maintained even though the expected value of currents to be sensed by the sensor may change depending upon the load and other circuit characteristics.

CROSS-REFERENCE TO RELATED APPLICATION

Certain inventions related to those disclosed in the present applicationare disclosed and claimed in copending application Ser. No. 504,404filed concurrently by W. Gary and G. R. Taylor, and copendingapplication Ser. No. 405,198 filed Oct. 10, 1973 by Wardell Gary andEmroy W. Lange, all of which are assigned to the same assignee as thepresent application.

BACKGROUND OF THE INVENTION

This invention relates generally to the electrical control systems forcontrolling the action of a circuit breaker and it relates specificallyto modular control systems utilizing plug-in modules which areuniversally adaptable for use over a wide range of current values.

In the past it has been known to provide circuit breaker control systemsutilizing a multitude or a number of current and voltage sensors tocontrol functions such as inverse time overload to thereby cause acircuit breaker to trip. Sometimes a separate circuit breaker isprovided for each function to be controlled. Sometimes it is necessaryto provide a multitude of current sensors each adapted to sensedifferent ranges of currents or different values of currents ordifferent rates of change of currents or voltage to in turn supply thatinformation to a logic device which in turn can cause a certain circuitbreaker to trip. Devices of this kind are described in U.S. Pat. No.3,713,005 entitled "Circuit Breaker Including Improved OvercurrentProtective Device" issued on Jan. 23, 1973 to J. C. Engel and assignedto the same assignee as the assignee of the present invention, and intechnical bulletin 980 of June 1972 entitled "Phase Failure Relays" bythe Wilmar Electronics, Inc. of 2103 Border Avenue, Torrance, Californiaand in a technical bulletin 948-B1 of June 1971, entitled "OverloadRelays" by the Furnas Electric Company of Batavia, Illinois. It would beadvantageous if a universal control system for a circuit breaker couldbe found which is utilizable over a wide range of circuit currents andvoltage conditions and which is adaptable to utilize plug-in logicmodules to control or cause the circuit breaker to trip in response to avariety of different kinds of circuit functions.

SUMMARY OF THE INVENTION

In accordance with the invention an electrical circuit protective deviceis taught having a sensor means for sensing circuit current in anelectrical circuit and providing an output current related to thecircuit current. There is included a replaceable load resistor meansconnectable to the last-mentioned output for converting the current intoa voltage the value of which is variable within a predetermined rangeregardless of the value of the circuit current. There is also provided areplaceable module which is connectable in parallel circuit relationshipwith the load resistor means which is capable of initiating a circuitbreaker trip function. The module is operable over the predeterminedrange of voltage. There is also provided a circuit breaker trip meanswhich is connected to the circuit module for opening the electricalcircuit when the trip function occurs in the module.

In another embodiment of the invention, more than one of the previouslydescribed replaceable modules is provided in parallel circuitrelationship with the load resistor means and with each other. Thepreviously described modules may comprise an inverse time overloadmodule, an instantaneous overcurrent module, a phase failure module, anunderload logic module, a phase imbalance module, and in anotherembodiment of the invention a field test panel or an overload conditionindicator, the latter two modules not having control over the circuitbreaker but providing an indication of the status of the electricalcircuit to be controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention reference may be had to thepreferred embodiment exemplary of the invention, shown in theaccompanying drawings in which:

FIG. 1 shows a universal plug-in type control system for a circuitbreaker for a three-phase electrical system;

FIG. 2 shows a system similar to that shown in FIG. 1 but for aone-phase electrical system;

FIG. 3 shows a circuit block diagram of a portion of the system shown inFIG. 1;

FIG. 4 shows a curve of the characteristic of a portion of the systemshown in FIG. 3; and

FIG. 5 shows a physical interconnecting plan for the apparatus shown inFIGS. 1 and 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings and FIG. 1 in particular, a circuitprotective system 10 is shown. The circuit protective system 10comprises in this embodiment of the invention a three-phase line havingleads, conductors or lins L1, L2 and L3 which are connected on the rightto a three-phase load and which are connected on the left to athree-phase source of electrical power. Intermediate to the load and thesource of electrical power is a current sensor 12 and a seriallyconnected circuit breaker or motor starter appartaus 45. In theembodiment of FIG. 1 a single current IL is shown flowing in the lineL1. It is to be understood that other currents may and usually do flowin the other lines L2 and L3 and the other currents may be related tothe current IL. The choice of current IL is merely made for the purposeof simplicity of illustration.

There are two output terminals for the current sensor 12, whichterminals are designated 14 and 16. Shown connected to the terminals 14and 16 is a load resistor module 18. The load resistor module 18comprises a resistive element which is connectable across the terminals14 and 16 to convert the current IL into a utilizable voltage V whichmay be utilized by other circuit protective means in the apparatus ofFIG. 1. Connected in parallel circuit relationship with the loadresistor module 18 may be a long acceleration module 20, an inverse timeoverload logic module 22, an instantaneous overcurrent logic module 24,a phase failure logic module 26, an underload logic module 28, a phaseimbalance logic module 30, a field test panel 32, and a motor in reverseindicator 34 if the load to be protected includes a motor. Numerousother combinations of logic modules may be provided in the same parallelcircuit relationship as shown with respect to the elements 20 through 34of FIG. 1. The remaining elements would be connected to terminals 38 and36 for example. It is to be understood that any of the modules 20through 34 may be removed or replaced and other modules may be addedprovided the parallel circuit relationship with the load resistor module18 is maintained. Each of the previously described modules 22 through 30for example have an output terminal which is connectable to a line 40which in turn is connected to an output switch 42 which in turn isconnected to the previously described circuit breaker means or circuitinterrupter means 45. In the preferred embodiment of the invention, thefield test panel 32 and the motor in reverse indicator panel 34 have nooutput to the line 40. In the preferred embodiment of the invention thevoltage V at the output terminals 14 and 16 is proportional to thecurrent IL. If the expected raise of current IL becomes significantlylarge a different load resistor may be disposed across terminals 14 and16 to make the voltage between the terminals 14 and 16 approximately thesame even though the current IL is significantly larger. The same wouldapply if the rated current range IL is significantly lower. This meansthat the elements 20 through 34 need not be changed as they aresensitive only to the voltage V. It also means that the output switch 42need not be changed. In a typical embodiment of the invention the longacceleration module 20 will perform a function which will be describedhereinafter. The inverse time overload logic module 22 provides what istypically known as the I² t = K function as is well known in the art.The instantaneous overcurrent logic module 24 performs the instantaneoustripping function that is well known in the art and which is related toextremely high values of overload current or short circuit current. Thephase failure logic 26 provides an indication that one of the phases orlines L1, L2 or L3 has failed and provides adequate switching inaccordance therewith. The underload logic module 28 provides anindication that the load has dropped below what is considered to be asafe predetermined value of current IL. The phase imbalance logic module30 provides an indication and an automatic signal to the output switch42 if the currents flowing in the lines L1, L2 and L3 becomesignificantly disproportionate to one another. The field test panel 32provides an output indication of current IL and other useful outputfunctions. The motor in reverse indicator 34 provides a functiontypified by its name, namely an indication that motor which may beattached to the three-phase load is in a reverse wired polarity.

Referring now to FIG. 2 there is another protective device 10' shown forutilization where there is a single phase or DC load and source. In thisembodiment of the invention there is provided a single phase or DC lineL1' which provides power to a single phase or DC load on the right froma single phase or DC source on the left. There is also provided a singlecontact circuit breaker or motor starter apparatus 45' having a contactA therein for interrupting the current IL'. The current sensing means12' may be the same as shown in FIG. 1. The load resistor module 18' isdifferent from the load resistor module 18 shown in FIG. 1 if the rangeof current IL' is significantly different than the range of current ILshown in FIG. 1. However, the long acceleration module 20, the inversetime overload logic module 22, the instantaneous overcurrent logicmodule 24, the underload logic module 28, the field test panel 32, andthe motor in reverse indicator 34 are or may be all the same as thosecorresponding modules shown in FIG. 1. This demonstrates the versatileuse of the circuit protector apparatus. It will be noted that there isno phase failure logic module or phase imbalance logic module in thisembodiment of the invention as those functions are typical of polyphaseAC electrical apparatus. It will be also noted that the outputs of themodules 22, 24, and 28, for example, are connected to the line 40 whichin turn is an input to the output switch 42 which in turn controls theline 44 causing the circuit breaker 45' to be actuated.

Referring now to FIG. 3, there is shown an embodiment of the inventionfor use with a three-phase line having a three-phase supply andcontrolling a motor M which is a three-phase load. In this embodiment ofthe invention the electrical and electronic elements comprising thecurrent sensor 12, the load resistor module 18, the inverse timeoverload module 22, the output switch 42, the long acceleration module20 and the circuit breaker 45 are shown in schematic form. Also shown inblock diagram form are the previously described functional blocks 24,26, 28, 30, 32 and 34 as well as the interconnecting terminals 38 and36, the line 40 and the output line or lines 44. In this case, a currentIL flowing in the line L1 is sensed by a transformer T1 in the currentsensor 12. The resistor R1 shown in the module 18 comprises the load ormotor current range determining resistor previously described. It isacross this resistor that the output voltage V exists.

Resistors R9, R10 and capacitor C1 form the time delay network for theoverload trip switch comprising transistors Q3 and Q4. The timingcapacitor C1 is held at a discharged state until the motor is near anoverload condition by the full load sense switch comprising transistorsQ1 and Q2. The trip signal from the transistors Q3 and Q4 is held by theautomatic reset delay network comprising elements C3, R13, and R16. Theoverload relay 80 is equipped with manual reset, relay RA1. Relay RA1operates and is held on to prevent the motor starter 45 from actuating.The motor starter coil is controlled by the output series switchcomprising SCR Q7 and bridge B1. The output series switch SCR Q7 isnormally biased on by the reset control switch comprising transistors Q5and Q6. When a trip signal appears, the reset control switch is turnedoff for a fixed time period.

The DC voltage proportional to line current IL that appears across R1will be referred to in the following circuit description as the "inputvoltage".

Resistors R3 and R5 form a voltage divider that presents a fraction ofthe input voltage to base resistor R4 of transistor Q1. The inputvoltage corresponding to full load current may be 10 volts in apreferred embodiment of the invention. At input voltages below about 9.5volts, the voltage at the emitter of Q1 is at least 0.7 volts above thevoltage at the base thereof. Thus Q1 is biased on. The collector currentof transistor Q1 flows through resistor R6 and into the base oftransistor Q2. Transistor Q2 is therefore biased on, and time delaycapacitor C1 is held to about 0.8 volts above ground.

When the input voltage rises above approximately 9.5 volts, the voltageat the emitter of transistor Q1 cannot rise above 7.7 volts because theZener diode D14 clamps at about 8.4 volts (at the current levelspermitted by series resistor R12). When the voltage at the junction ofresistor R4, resistors R3 and R5 is not sufficiently below this lattervalue to allow Q1 to remain on and the collector current of Q1 ceasesflowing through base resistor R6 and into transistor Q2. Thus Q2 turnsoff, and timing capacitor C1 begins to charge through resistors R9 andR10. Resistor R7 prevents undesired turn-on of Q2 due to hightemperature reverse current leakage through the collector-base thereof.Diode D7 prevents C1 from being charged through resistors R12 and R8.Diode D8 prevents C1 from being robbed of charging current by theotherwise relatively low impedance path to ground of diode D7, resistorR8 and Zener diode D14.

When the full load sense switch Q1, Q2 turns off, the time delaycapacitor C1 begins to charge through resistors R9 and R10. The rate ofcharge depends on the value of the input voltage: the greater theoverload current IL, the faster capacitor C1 will charge. Trip signalswitch Q3 and Q4 uses Zener diode D14 as a reference voltage device. Aslong as the voltage at the emitter of Q3 is less than the base voltagethereof, Q3 remains off. Transistor Q4 is also off, and the trip signal(voltage across R11) is zero. When the voltage at the emitter of Q3(voltage across C1) exceeds by 0.7 volts the voltage on the base of Q3,the Q3 begins to turn on. Base-emitter current through Q4 begins to turnon Q4 and lower the collector-to-emitter voltage of Q4. The reducedvoltage at the junction of the collector of Q4 and base of Q3 causes Q3to turn on harder, thus producing the snap-action switch-on of thetransistor device comprising transistors Q3 and Q4. The energy normallystored in capacitor C2, which is charged through resistor R12 and diodeD9, is dumped or flows through Q4 by the sudden turn-on thereof. Most ofthe energy stored in C2 is dumped into two parallel paths: automaticreset delay capacitor C3, and relay coil RA1. Resistor R11 is relativelyhigh in impedance compared to the other two parallel paths, but providesa path to ground for Q4 when Q4 is normally off. Diodes D10 and D11isolate C3 and RA1 from each other.

When a trip signal charges reset delay capacitor C3, the reset controlswitch Q5, Q6 is turned off, and remains off until C3 discharges throughreset delay resistors R13 and R16 to a value of about 2 volts or less.The ouput series SCR, Q7, which is normally gated on is also turned offfor this time period.

Under normal conditions when 110 volts AC control voltage is applied tothe starter coil K, the series SCR Q7 is gated on every half cycle. Thefull wave AC voltage (rectified by B1) appears across the anode tocathode of SCR Q7. When the voltage at the anode of Q7 rises to 2 voltsor more, transistor Q5 turns on, provided the reset delay capacitor C3is discharged. The collector current of Q5 flows through thebase-emitter of Q6 and into the gate of SCR Q7. When SCR Q7 turns on,the anode-to-cathode voltage of Q7 drops to about 1.5 volts, and most ofthe AC voltage appears across the starter coil K.

When a trip signal has charged capacitor C3 to at least 3 volts or more,then at the beginning of the next half cycle, the base-emitter junctionof transistor Q5 is reverse biased and Q5 does not turn on. Thustransistor Q6 is turned off, and no gate current is supplied to SCR Q7.As the AC voltage continues to rise, when the voltage at the emitter ofQ5 reaches about 21/2 to 3 volts, light-emitting diode or LED D13 andseries diode D12 conduct. This prevents the voltage at the emitter of Q5from rising further, and Q5 thus remains off. During the remainingportion of each half cycle, the voltage across Q7 continues to rise andthen fall. This provides enough current through R19 and D13 to produce avisible indication that the overload 80 has caused starter 45 to open ortrip. If the AC control voltage to the starter coil K is removed whenthe overload relay 80 trips (as in the case when the starter 45 isoperated by a pushbutton and auxiliary contact on the starter, notshown), there will be no available voltage to operate the light emittingdiode D13. When the start button is pushed, however, if the overloadrelay 80 is still in a tripped condition the LED D13 will turn on andilluminate.

Resistor R15 limits the collector current of Q6 to a reasonably lowvalue, and resistor R14 prevents undesired turn-on of Q6 due to hightemperature reverse leakage current through the collector-base of Q6.Resistor R17 helps prevent undesired turn-on of SCR Q7 due to hightemperature leakage or transient noise. Resistor R18 and capacitor C4provide a snubber network to protect SCR Q7 against actuation thereof byexcessive dv/dt.

If the circuit is equipped with a manual reset, then relay coil RA1 isenergized by the current through D12 and D13 and also by the trip signalthrough D10. If the voltage to the starter coil K is applied through apushbutton and an auxiliary contact of the starter (not shown), thevoltage applied to the SCR Q7 could be removed too soon to energize thecoil RA1 in the event the auxiliary contact operates too quickly. Thiscondition would cause the reset circuit to operate in the automaticmode, and the motor M could be restarted in a few minutes by pushing thestart button, not shown (without requiring operation of the resetbutton). For this reason RA1 is energized by both the trip signalthrough D10 and the current supplied through R19, D13 and D12.

When the relay coil RA1 is actuated, the contacts close and short thegate of SCR Q7 to the cathode, turning Q7 off. If the reset delaycapacitor C3 has discharged, the output series SCR Q7, will turn onagain when voltage is re-applied to the starter coil K. If the manualreset mechanism is operated before the reset delay network has timedout, the relay RA1 contacts will open, but they will be reclosed by thecoil of RA1 if the start button (not shown) is pushed before the resetdelay time had elapsed. The manual reset mechanism (not shown) must thenbe operated again before the start button can actuate the starter 45. Ifthe manual reset mechanism is operated before the reset delay has timedout, but the start button is not pushed until after the reset delay timehas elapsed, then the starter 45 will operate. In any event, the starter45 cannot be energized until three conditions are met: the manual resetmechanism has been operated at least once; the reset delay time hasexpired; and the start button (not shown) is pushed or actuated.

It can be seen that the trip signal is provided by way of line 40 to theoutput module 42 and then by way of lines 44 to the circuit breakerapparatus 45 where the contacts A, B and C are opened under appropriateconditions. It can be seen that any of the devices 20, 22, 24, 26, 28and 30 can provide an output signal which can independently provide asignal on line 40 to cause tripping.

Referring now to FIG. 4, a plot of percent motor full load currentversus trip time in seconds for the apparatus of FIG. 3 is shown. Undernormal conditions the trip time versus percent motor full load currentfollows line 50. However, the utilization of a Zener diode 20 connectedbetween the terminals 14 and 16 allows for what is generally called along acceleration characteristic. This means that a motor or otherdevice which takes a long period of time to accelerate where overloadcurrent such as IL may therefore exist for a long period of time willnot necessarily cause tripping of the circuit breaker apparatus 45.Other circuit breaker apparatus not shown and interconnected to otherportions of the lines L1, L2 and L3 will provide protection for severeoverload.

Depending upon the characteristics of the Zener diode 20, the time whichis allowed for the acceleration of the motor into a fairly high overloadcondition may be varied. As an example, if a Zener diode 20 is chosenwhich corresponds to line 52 of FIG. 4, a full 40 seconds of motoracceleration in the overload range may be allowed without the trippingof the circuit breaker 45. On the other hand if the Zener diode 20 ischosen with the characteristic 54 shown in FIG. 4, then a limitedacceleration time of 20 seconds is allowed for the motor to reach speedbefore a tripping operation will occur. Also as an example, if the Zenerdiode 20 is chosen with the characteristic 56 shown in FIG. 4, then only15 seconds for acceleration is allowed. The Zener diode 20 can bereplaced in the field according to the overload characteristics of theapparatus being protected by the system 10" shown in FIG. 3.

Referring now to FIG. 5, the packaging concept utilizing the inventionis shown. In this case the three-phase lines L1, L2 and L3 are shownconnectable to a three-phase load on the right (not shown) and athree-phase source on the left (not shown). A module 12-42 whichcomprises the current sensors 12 and the output switch 42 is providedand interconnected with the lines L1, L2 and L3. Terminals 40, 16 and 14are provided, the functions of which have been described previously withrespect to the other figures. A plug-in module such as 22 whichcorresponds to the inverse time overload logic module shown in FIGS. 1,2 and 3 is shown having plug-in pins interconnectable with theconnectors 40, 16 and 14 of the module 12-42. A second plug-in modulewhich may comprise the instantaneous overcurrent logic module 24 isinterconnectable with other pins 40, 38 and 36 which may be on the backpart of the previously described module 42. As can be seen the plug-inmodules 22 and 24 may be disconnected or interplaced with each other.The module 12-42 has a set of output terminals 44 which correspond tothe line 44 shown in FIGS. 1, 2 and 3. To this line may be connected acircuit breaker, not shown, but which is generally designated as 45 or45' in FIGS. 1, 2 and 3.

It is to be understood that with respect to the embodiments of thisinvention that other modules than those shown in FIGS. 1, 2 and 3 may beprovided at terminals 38 and 36. It is also to be understood that thiscircuit protective concept may be utilized with multiphase or directcurrent protective apparatus. It is also to be understood that the motorM shown in FIG. 3 is not limiting. It is also to be understood that thecurves 52, 54 and 56 shown in FIG. 4 are not limiting and that otheroperating characteristics may be utilized depending upon the choice ofthe Zener diode means 20.

The apparatus taught in this invention has many advantages. Oneadvantage lies in the fact that the apparatus may be utilized over awide range of operating characteristics which may include full ratedcurrents which vary significantly from apparatus to apparatus. Anotheradvantage lies in the fact that the apparatus may be changed in thefield or reprogrammed in the field by replacing the load resistor 18.Another advantage lies in the fact that if any of the operating modulesfail, that module may be replaced without having to replace the entiresystem. Another advantage lies in the fact that devices such as motorswhich may take long periods of time to reach normal speed after start,may be utilized without causing an unnecessary tripping of the circuitbreaker or motor starter 45 or 45' if the means 20 shown in FIGS. 1, 2and 3 is utilized.

What we claim as our invention is:
 1. An electrical circuit protectingdevice, comprising:sensor means for sensing circuit current in anelectrical circuit, said sensor means having an output for providing anoutput current related to said circuit current; replaceable loadresistor means connected to said output of said sensor means forconverting said output current into a voltage the value of which isvariable within a predetermined voltage range for a predetermined rangeof said circuit current; a replaceable module connected in parallelcircuit relationship with said load resistor means, said module beingcapable of initiating a circuit breaker trip function, said module beingoperable over said predetermined range of said voltage; and circuitbreaker trip means connected to said module for opening said electricalcircuit when said trip function occurs in said module.
 2. Thecombination as claimed in claim 1 wherein there are a plurality of saidmodules each of which is connectable in parallel circuit relationshipwith said load resistor means and with each other, each of which iscapable of independently initiating said trip function.
 3. Thecombination as cliamed in claim 1 wherein there is a second replaceablemodule which is capable of performing an indicating function, saidsecond module being operable over said predetermined range of saidvoltage.
 4. The combination as claimed in claim 1 wherein saidreplaceable module comprises an inverse time overload module.
 5. Thecombination as claimed in claim 1 wherein said replaceable modulecomprises an instantaneous overcurrent module.
 6. The combination asclaimed in claim 1 wherein said replaceable module comprises a phasefailure module.
 7. The combination as claimed in claim 1 wherein saidreplaceable module comprises an underload logic module.
 8. Thecombination as claimed in claim 1 wherein said replaceable modulecomprises a phase imbalance module.
 9. The combination as claimed inclaim 3 wherein said second replaceable module comprises a field testpanel.
 10. The combination as claimed in claim 3 wherein said secondreplaceable module comprises an overload condition indicator.
 11. Anelectrical motor starter comprising:sensor means for sensing circuitcurrent in an electrical circuit, said sensor means having an output forproviding an output current related to said circuit current; replaceableload resistor means connected to said output of said sensor means forconverting said output current into a voltage the value of which isvariable within a predetermined voltage range for a predetermined rangeof said circuit current; a replaceable module connected in parallelcircuit relationship with said load resistor means, said module beingcapable of initiating a circuit breaker trip function, said module beingoperable over said predetermined range of said voltage; and circuitbreaker trip means connected to said module for opening said electricalcircuit when said trip function occurs in said module.